This invention relates to digital communications apparatus and methods, and more particularly to apparatus and methods for filtering channel taps of a signal in a communication channel.
Digital communications systems are widely used for voice, image and data communications. When communicating wirelessly, time-varying multipath fading of wireless channels and co-channel interference among wireless channels can make such communications unreliable. Equalization may be used to compensate for the effects of multipath fading and co-channel interference. Some types of equalizers include Maximum Likelihood Sequence Estimation (MLSE), Decision Feedback Equalization (DFE), and Decision Feedback Sequence Estimation (DFSE), reduced-state sequence estimation (RSSE), and a M-algorithm technique.
Multiple-input-single-output (MISO) and Multiple-input-multiple-output (MIMO) channels arise in many communication applications. A MISO channel a may arise when a receiver performs joint detection on co-channel signals using a single antenna. A MIMO channel may arise when a receiver uses antenna diversity, or uses one or multiple antennas for interference rejection, and/or performs multiuser demodulation and/or multicarrier detection.
Equalization of MIMO channels can be particularly difficult because receiver complexity can grow exponentially with the number of channel impulse response taps (or channel memory) of a received signal. A pre-filter may be used to shape or shorten the channel impulse response taps, which may allow the use of an equalization technique that has a lower complexity than MLSE, such as, for example, DFE, DFSE, RSSE, and the M-algorithm technique.